The present invention relates generally to dual frequency laser and laser array systems. More specifically, the present invention relates to dual frequency laser and laser array systems for selectively generating first and second frequency laser beams. Methods of operating the dual frequency laser and laser array systems are also disclosed.
It is generally known that the emission wavelength of a fiber optic laser amplifier or a fiber optic phased array source advantageously can be set by the wavelength of a master oscillator, provided that wavelength is within the gain bandwidth of the rare earth dopant in the fiber amplifier single mode core. Dopant ions commonly used in fiber amplifiers include Nd, Yb, Er, Pr, Tm and Ho. It will be appreciated that the gain bandwidth for all of these dopants is from a few to several tens of nanometers wide, e.g., from 1050 nm to 1080 nm for Nd and from 1530 nm to 1565 nm for Er. It would be highly advantageous to be able to operate a fiber optic laser amplifier at more than one of the emission wavelength bands associated with these rare earth dopants. It would be particularly advantageous to be able to operate a fiber optic laser amplifier constructed, at least in part, using a co-doped optical fiber, at more than one of the emission wavelength bands associated with these rare earth dopants.
In order to generate the desired first and second laser beams at first and second frequencies, respectively, several criteria must be satisfied. First, a laser cavity or optical fiber capable of producing the required lasing action must be available. As will be discussed in greater detail below, co-doped optical fibers, e.g., Yb:Er doped optical fibers, have been produced but have been employed in markedly different systems. Second, the laser cavity or optical fiber (array) must be operated under appropriate drive conditions.
Co-doped amplifying media are generally known. For example, U.S. Pat. No. 4,701,928 discloses a diode laser pumped co-doped laser having a YAG host crystal doped with both Ho and Er. In this patent, the Er functions as an absorber ion and the Ho as the laser ion, whereby the Er absorber ion absorbs the pump radiation provided by one or more laser diodes and transfers the absorbed radiation to the Ho laser ion for inverting the population of the desired energy transition levels to produce an eyesafe output beam. U.S. Pat. Nos. 5,172,387 and 5,182,759 disclose variants of this operating principle. An additional variant is disclosed in U.S. Pat. No. 5,566,196, wherein singly doped optical fibers are bundled in close proximity to one another, allowing interaction between the differently doped optical fibers. It should be mentioned that all of these patents are incorporated herein by reference.
Co-doped optical fibers wherein two of the dopants have been implanted into a single mode core of an amplifier fiber are also known. For example, Yb:Er co-doped optical fibers have actually been produced, as disclosed in U.S. Pat. No. 5,594,747, which patent will be discussed in greater detail below. It will be appreciated that it is difficult to pump Er fibers because absorption is generally weak across the spectrum except in a narrow band centered either around 980 nm, for which pump diodes are neither adequately developed nor readily available, or around 1480 nm, for which adequate laser pump diodes are also not available. In contrast, excellent pump diodes are available in the wavelength region of 915 nm to 940 nm for pumping Yb doped fiber amplifiers, which normally provide gain in the range of 1030 nm to 1100 nm, i.e., the 1 .mu.m band. Efficient transfer of energy has been obtained from excited Yb to Er ions, making it possible to pump Yb:Er fibers at 925 nm and produce gain at the 1.5 .mu.m band.
U.S. Pat. No. 5,594,737, which is incorporated herein by reference, discloses a dual-wavelength pumped low noise fiber laser including a fiber laser 10 comprising a pair of Bragg gratings 14, 16 at opposite ends of a fiber laser cavity 18 which is co-doped with two rare-earth dopants, Er.sup.+3 and Yb.sup.+3, which allows lasing at a lasing wavelength .lambda..sub.L. A first pump signal 20 at a first wavelength .lambda..sub.P1 efficiently pumps the Yb to the excited state and the Yb energy is transitioned to the Er atoms which ultimately lase at the desire lasing frequency. Simultaneously, a second pump signal 52 directly pumps the Er at a different wavelength .lambda..sub.P2, which populates the lasing transition more quickly.
As shown in FIG. 1, a conventional dual-wavelength pumped low noise fiber laser 10 includes a fiber laser 12 having Bragg gratings 14,16 embedded in the core of the fiber a predetermined distance apart. Between the gratings 14,16 is a region of fiber 18 doped with two predetermined rare-earth dopants (or gain or active medium), e.g., Erbium (Er.sup.+3) and Ytterbium (Yb.sup.+3), which acts as a laser cavity 18. The gratings 14,16 have a grating spacing which provides a peak reflectivity at a lasing wavelength .lambda..sub.L, e.g., about 1550 nanometers, for an erbium-doped cavity, of the fiber laser. The gratings 14,16 and the doped fiber cavity 18 make up the three fundamental elements of a typical fiber laser.
Furthermore, the fiber laser 12 is pumped by a first input pump light 20 from a first pump light source 22, e.g., a laser diode. The pump light 20 has a first pumping wavelength .lambda..sub.P1, e.g., 980 nanometers. The pump light 20 travels along a fiber 24 to a port 26 of a known wavelength division multiplexer (WDM) 28 which provides wavelength sensitive coupling of light. The light 20 is coupled to a port 30 of the WDM 28 and propagates along an optical fiber 32 to the fiber laser 12. The pump light 20 passes through the grating 16 and enters the laser cavity 12. The pump wavelength .lambda..sub.P1, primarily excites the Ytterbium (Yb) portion of the gain medium of the cavity 18 to a predetermined energy level. The energy of the excited Yb atoms transitions to the Erbium (Er) atoms and the Er emits photons at the lasing wavelength .lambda..sub.L, as indicated by a line 36. The light 36 at the lasing wavelength .lambda..sub.L that passes through the back grating 16 exits the laser as output laser light 40 along the fiber 32. Moreover, a predetermined amount of the cavity light 36 reflects off the grating 16, as indicated by a line 44. The light 44 is incident on the grating 14 which reflects the aforementioned predetermined amount of light at the lasing wavelength .lambda..sub.L and passes a portion of the light 46 out of the cavity 18 at the lasing wavelength .lambda..sub.L along a fiber 48.
For lasing to be sustained, the lasing condition (or lasing threshold) must be met, i.e., the round trip small signal gain times the round trip loss for light within the cavity is greater than or equal to one. This is accomplished by setting the amount of cavity gain, the length of the cavity, and the reflectivity of the gratings so as to meet this condition. If the product of these factors is greater than one, laser beam power will build up to the saturation gain limit of the amplifier. It will be understood that while the length of the cavity is not critical, for single longitudinal mode operation, the laser cavity should be as short as possible.
The laser 12 is also pumped by a second optical signal 52 from an optical pump source 54, e.g., a laser diode, having a second pump wavelength .lambda..sub.P2, e.g., 1480 nm. The source 54 provides the second pump signal 52 along a fiber 56 to a port 58 of a WDM 60, which is similar to the WDM 28. The WDM 60 couples the light 52 to a port 62 of the WDM which is coupled to the fiber 48. The pump light 52 passes through the grating 14 and enters the cavity 18. The pump light 52 excites the Er portion of the Yb:Er gain medium and allows the Er to emit lasing light at the lasing wavelength .lambda..sub.L e.g., 1550 nm). As discussed hereinbefore, light within the cavity at .lambda..sub.L resonates in the cavity between the reflectors 14,16 and a portion is allowed to pass as output light signals 46,40, respectively, from the laser.
The light 46 enters the port 62 of the WDM 60 which couples the light 46 at the lasing wavelength .lambda..sub.L to a port 64 of the WDM 60. The light 46 travels along an optical fiber 66 and is fed to an optical detector 68 which detects the light 46 and provides an electrical signal on a line 70 indicative thereof. The line 70 is fed to a control circuit 72 which provides an electrical signal on a line 74 to the pump source 54. The control circuit 72 contains known electronic control components (e.g., op-amps, etc.) designed to provide proportional-integral-differential (PID) closed loop control of the intensity of the output light from the laser. Alternatively, the control circuit 72 can be replaced by a known computer with comparable known control software.
The light 40 that exits the fiber laser 12 includes both light at the lasing wavelength .lambda..sub.L and light at the pump wavelength .lambda..sub.P2 that was not absorbed by the gain medium in the fiber laser 12. The light 40 is fed to the port 30 of the WDM 28 along the fiber 32. The WDM 28 couples the light 40 to a port 76 of the WDM 28 and exits the WDM on a fiber 78, and is the output light from the dual-wavelength pumped fiber laser 10.
The light 40 from the WDM 28 passes through an optical isolator 82, which passes light in only one direction, and is incident on a fiber optic amplifier 84. The amplifier 84 comprises an optical fiber which is doped with a rare-earth dopant (or gain medium), e.g., erbium, and provides an output light 86 which is amplified at the lasing wavelength .lambda..sub.L from that of the input signal 40. The light 86 passes though an output isolator 88, which passes light in only one direction, and then travels along a fiber 90 and ultimately exits the fiber 90 as indicated by a line 92. The amplifier 84 uses the remaining pump energy at the second pump wavelength .lambda..sub.P2 (that was not absorbed by the gain medium of the fiber laser 12) to excite the amplifier gain medium to a level that allows the stimulated emission of photons at the lasing wavelength .lambda..sub.L by the amplifier 84. It will be appreciated that the isolator 82 prevents light 94 emitted by the amplifier 84, toward the WDM 28, from entering and disrupting the operation of the fiber laser 12. Moreover, the isolator 88 prevents external optical signals from entering and disrupting the operation of the amplifier 84.
In short, U.S. Pat. No. 5,594,747 discloses a system capable of operation only at the 1550 nm Er wavelength by pumping at the broad Yb absorption bands with subsequent energy transfer by cross relaxation. The disclosed system is incapable of operating at other predetermined wavelengths, i.e., is incapable of satisfying the second criteria.
What is needed is a dual laser amplifier system and operating method therefor capable of selectively generating at least two output laser beams at predetermined first and second wavelengths. Moreover, what is needed is a dual frequency laser system and operating method therefor capable of operating as either an amplifier on an oscillator. It would be highly desirable to include provisions in the dual laser amplifier system and operating method therefor whereby the system and corresponding operating method could be operated only at eyesafe wavelengths during training exercises.